Retroreflectors are optical elements that can direct a significant fraction of light incident upon them back towards the source. There are two distinct regimes of interest with regards to retroreflector performance within an optical system. In the first regime, incident light is within a beam considerably smaller than the retroreflector in terms of transverse size, and only a single retroreflector is used. In this regime, one can expect that either 100% of the incident light or none is reflected back towards the source, depending upon whether the incident beam is within the corner cube's angular acceptance. These values are intended as illustrative only and do not include real-world effects, such as scattering of light from dust on the surface of the retroreflector, an optical coating that absorbs a portion of the incident light, etc. An example of this regime would be the use of a corner cube as part of an interferometer or similar optical instrument.
In the second regime, the incident optical beam has a transverse size equal to or larger than the retroreflector. Many retroreflectors may be used, in either a specific or random orientation, as part of the optical system to form a larger retroreflective surface or volume. However, the incident light still covers a larger area than any single retroreflective element. In this case, the incident beam “fills” the retroreflector(s), and the fraction of incident light reflected back towards the source is a more or less a smooth function of the orientation of the retroreflector(s) wholly or partially within the light beam. The second regime that is considered herein.
Arguably, the simplest retroreflector is the corner cube, which consists of three reflective plane surfaces. Light rays entering the corner cube's acceptance window are reflected back towards their source along parallel but potentially offset trajectories. FIG. 1 is a front perspective view illustrating a corner cube retroreflector 100. Corner cube retroreflector 100 can essentially be thought of as a cube with three of its six sides removed. From this perspective, corner cube retroreflector 100 is “indented,” where back face 110, bottom face 120, and side face 130 form a recess with respect to the viewer. For a corner cube to return light towards its source, the source must be facing the “indented” (i.e., concave) side of the corner cube.
Given at least approximate prior knowledge of an optical system's configuration, corner cubes can be highly effective retroreflectors. For example, on a highway, the direction of car travel (and thus, the orientation of car headlights) is generally well known. Thus, corner cube-based retroreflectors can be highly effective at directing incident light back in the direction of the source (e.g., to increase a road sign's visibility to a driver). This type of corner cube can, in principle, be made easily from sheet metal via a stamping process.
There are situations, however, in which the orientation of an optical system is either not known or cannot be assumed to be static beforehand. In such cases, a retroreflector capable of returning light with a wider angular acceptance than a conventional corner cube is desirable. The most straightforward design of such a retroreflector may be a combination of eight corner cubes, joined at their inner vertices. As an alternate description, consider three orthogonal planes bisecting a cube and intersecting at the middle of the cube. This arrangement is shown in FIG. 2, and may be referred to as a “multiplane cross” retroreflector. Multiplane cross retroreflector 200 includes eight joined corner cubes, such as corner cube 240 with faces identified by plus signs. The eight corner cubes are defined by planes 210, 220, 230.
Multiplane cross retroreflectors have the advantage of being able to return at least some fraction of incident light back towards the source, regardless of the orientation of the retroreflector relative to the source. However, fabrication is more complex, as this shape cannot be simply stamped from a sheet. Also, multiplane cross retroreflectors cannot be packed efficiently into a small volume. Rather, they will be mostly “empty space.” In comparison, conventional corner cubes made from stamped sheets can be densely packed, and so fit more retroreflectors into a given volume. However, conventional corner cubes do not work well for unknown or non-static optical systems. Furthermore, while retroreflectors have been used as part of “passive” systems (e.g., the Apollo missions left retroreflectors on the moon's surface that have been used to precisely measure the distance between the Earth and the moon using Earth-based lasers, and some security systems use retroreflectors for laser “tripwires”), these systems are not a communications system in any real sense as the “far end” doesn't provide any information back other than the retroreflection itself. Retroreflectors have not previously been used as part of an active two-way communication system. Accordingly, an improved retroreflector and retroreflector-based communication system may be beneficial.